1. Field of the Invention
The present invention relates to an electron-emitting device for emitting electrons by the application of a voltage, an electron source and an image-forming apparatus employing the electron-emitting device.
2. Related Background Art
Conventionally, two types of electron sources, a thermionic cathode electron source and a cold cathode electron source are known as an electron-emitting device. The cold cathode electron source includes a field emission (hereinafter referred to as FE) electron-emitting device, a metal/insulator layer/metal (hereinafter referred to as MIM) electron-emitting device, a surface conduction electron-emitting device and the like.
As examples of the FE electron-emitting device, those disclosed in W. P. Dyke and W. W. Dolan, xe2x80x9cField Emissionxe2x80x9d, Advance in Electron Physics, 8, 89 (1956), C. A. Spindt, xe2x80x9cPhysical Properties of thin-film field emission cathodes with molybdenium conesxe2x80x9d, J. Appl. Phys., 47,5248 (1976) and the like are known.
As examples of the MIM electron-emitting device, those disclosed in C. A. Mead, xe2x80x9cOperation of Tunnel-Emission Devicesxe2x80x9d, J. Apply. Phys., 32,646 (1961) and the like are known.
In addition, in recent examples, Toshiaki Kusunoki, xe2x80x9cFluctuation-free electron emission from non-formed metal-insulator-metal (MIM) cathodes fabricated by low current anodic oxidationxe2x80x9d, Jpn. J. Appl. Phys. Vol. 32 (1993) pp. L1695, Mutsumi Suzuki et al., xe2x80x9cAn MIM-Cathode array for Cathode Luminescent Displaysxe2x80x9d, IDW ""96, (1996) pp. 529 and the like are studies.
As examples of the surface conduction electron-emitting device, there are those described in an Elinson report (M. I. Elinson, Radio Eng. Electron Phys., 10 (1965)) or the like. This surface conduction electron-emitting device utilizes a phenomenon that electron emission is caused by flowing a current to a thin film with a small area, which is formed on a substrate, in parallel with the surface of the film.
As the surface conduction electron-emitting device, one using an SnO2 thin film described in the above-mentioned Elinson report, one using an Au thin film (G. Dittmer, Thin Solid Films, 9,317 (1972)), one using an In2O3/SnO2 thin film (M. Hartwell and C. G. Fonstad, IEEE Trans. ED Conf., 519 (1983)) and the like are reported.
However, in the case of the above-mentioned conventional art, there are problems as described below.
In order to apply an electron-emitting device to an image-forming apparatus, an emission current for causing a phosphor to emit light with sufficient luminance is required. In addition, a diameter of an electron beam irradiated on a phosphor is required to be small for making a display high in definition. Moreover, it is important that the electron-emitting device can be manufactured easily.
As an example of the conventional FE electron-emitting device, a Spindt type electron emitting-device is shown in FIG. 13. In FIG. 13, reference numeral 1 denotes a substrate, 4 denotes a cathode electrode layer (low potential electrode), 3 denotes an insulating layer, 2 denotes a gate electrode layer (high potential electrode), 5 denotes a microchip and 6 denotes an equipotential surface.
When a bias is applied between the microchip 5 having a curvature r and the gate electrode layer 2, electrons are emitted from the tip of the microchip 5 and heads for an anode. An amount of the emitted electrons is determined by a work function of a distance d between the gate electrode layer 2 and the tip of the microchip 5, a gate voltage Vg and a material of an emitting portion. That is, it is an element for determining performance of a device to manufacture it with good control of the distance d between the gate electrode layer 2 and the microchip 5.
A general manufacturing process of the Spindt type electron-emitting device is shown in FIG. 14.
The manufacturing process will be described along this figure. First, the cathode electrode layer 4 made of Nb or the like, the insulating layer 3 made of SiO2 or the like and the gate electrode layer 2 made of Nb or the like are stacked in this order on the substrate 1 made of glass. Thereafter, a circular opening (fine hole) perforating through the gate electrode layer 2 and the insulating layer 3 is formed by a reactive ion etching method (FIG. 14A).
Then, a sacrifice layer 7 made of aluminum or the like is formed on the gate electrode layer 2 by diagonal evaporation or the like (FIG. 14B).
A microchip material 8 such as molybdenum is deposited by a vacuum evaporation method on the structure formed as described above. Thus, the deposit on the sacrifice layer 7 stuffs the inside of the opening as deposition advances, and the microchips 5 are formed in a conical shape inside the opening (FIG. 14C).
Lastly, the sacrifice layer 7 is dissolved, whereby the microchip material 8 is lifted off to complete a device (FIG. 14D).
However, it is difficult for such a manufacturing method to manufacture the device with good control of the above-mentioned distance d, and an amount of emission current varies for each device. In addition, it is likely that a metal piece or the like generated during the lift-off causes short-circuit of the microchip 5 and the gate electrode layer 2. In this case, if a voltage is applied between the microchip 5 and the gate electrode layer 2 at the time of driving, heat is generated in a shorted part to cause destruction of the shorted part and the part around it. Thus, an effective emission area is reduced.
If the electron-emitting device is applied as, for example, an image-forming apparatus, variation of an amount of emission current for each device causes unevenness of luminance, which makes an image extremely unsightly.
Moreover, in this example, since electrons are emitted from one emitting point, if a density of emission current is increased in order to cause the phosphor to emit light, thermal destruction of the electron-emitting region is induced, which limits a life of the FE device. In addition, ions existing in the vacuum intensively may sputter the tip of the microchip, thereby reducing the life of the device.
Further, electrons emitted into the vacuum usually advances perpendicular to the equipotential surface 6. However, in the configuration as shown in FIG. 13, since the equipotential surface 6 is formed in a hole along the microchip 5, electrons emitted from the tip of the microchip 5 tend to spread.
In addition, when spreading is caused in the emitted electrons in this way, a part of the emitted electrons is absorbed in the gate electrode layer 2, whereby an amount of electrons reaching the anode is reduced. The amount of electrons absorbed in the gate electrode layer 2 tends to increase as the distance d is reduced.
In order to overcome such drawbacks of the FE device, various examples have been proposed as individual solutions.
As an example for preventing spreading of electron beams, there is an example in which focusing electrodes 9 are disposed above the electron-emitting region as shown in FIG. 15. FIG. 15 is a view showing a configuration of an FE device with focusing electrodes. In this example, emitted electron beams are focused by a negative potential of the focusing electrodes 9. A process that is more complicated than the above-mentioned manufacturing process is required in this example, which causes increase in manufacturing costs.
As an example of reducing an electron beam diameter without disposing a focusing electrode, there is a method that does not involve formation of a microchip such as the Spindt type. For example, there are technologies disclosed in Japanese Patent Application Laid-open No. 8-096703, Japanese Patent Application Laid-open NO. 8-096704, Japanese Patent Application Laid-open No. 8-264109, U.S. Pat. No. 5,939,823, U.S. Pat. No. 5,989,404 and the like.
These disclosed technologies have advantages that flat equipotential surfaces are formed on an electron emitting surface and spreading of electron beams becomes smaller because electrons are emitted from a thin film disposed in a hole.
In addition, there are also advantages that, since a material with a low work function is used as an electron emitting substance, electrons can be emitted without forming a microchip, driving is allowed at a low voltage and a manufacturing method is relatively simple.
Moreover, there are also advantages that, since electrons are emitted on a plane, concentration of electric fields does not occur, destruction of chips is not caused and a length of life will be longer.
However, since a distribution of potentials correlated to a depth of the hole and a distance between gate electrode layers is formed around the hole in these examples, emitted electrons still tend to spread, although not so widely as in the Spindt type. Thus, the problem that a part of the emitted electrons is absorbed in the gate electrode layer 2 or scattered has not been solved.
As an example of improving an electron emission efficiency, there is, for example, a technology disclosed in Japanese Patent Application Laid-open No. 10-289650 as shown in FIG. 16.
In this technology, a device has a structure in which the gate electrode layer 2 and a second electrode layer 11 are provided on both sides of the cathode electrode layer 4 via the insulating layers 3, respectively.
Then, a positive potential is applied (provided that 0 less than |Vg1|xe2x89xa6|Vg2|) to the gate electrode layer 2 and the second gate electrode layer 11 with respect to the cathode electrode layer 4, whereby an amount of electrons emitted from the cathode electrode layer 4 is increased. However, the emitted electrons still tend to spread.
On the other hand, the MIM electron-emitting device has a structure in which the insulating layer 3 is disposed between a lower electrode (cathode electrode layer 4) and an upper electrode (gate electrode layer 2) as shown in FIG. 17 to apply a voltage between both the electrodes and take out electrons.
In the case of this structure, since a direction of an internal electric field and a direction of emitted electrons coincide with each other and there is no distortion in a distribution of potentials on the emission surface, a small electron beam diameter can be realized. However, efficiency is generally low because scattering of electrons occurs in the insulating layer 3 and the upper electrode.
A conventional example in which these electron-emitting devices are applied as an image-forming apparatus will now be described with reference to FIG. 18. FIG. 18 is a view illustrating the case in which an electron-emitting device in accordance with the conventional art is applied to an image-forming apparatus.
As shown in the figure, the image-forming apparatus constitutes a so-called triode device in which lines of the gate electrode layers 2 and lines of the cathode electrode layers 4 are arranged in a matrix shape, and electron-emitting devices 14 are disposed at intersections of both the lines. Electros are emitted from the electron-emitting device 14 of a selected intersection and accelerated by a voltage of an anode 12 to be incident in a phosphor 13 according to an information signal.
If it is considered that a field emission electron-emitting device is applied to the above-mentioned image-forming apparatus such as a display, the device is required to meet the following conditions:
(1) an electron beam diameter is small;
(2) an electron emission area is large;
(3) highly efficient electron emission is possible at a low voltage; and
(4) a manufacturing process is easy.
It is difficult to meet these conditions simultaneously using the conventional electron-emitting device.
The present invention has been devised in order to solve the above-mentioned problems of the conventional art, and it is an object of the present invention to provide a field emission electron-emitting device in which an electron beam diameter is small and an electron emission area is large, with which highly efficient electron emission is possible at a low voltage and whose manufacturing process is easy.
In order to attain the above-mentioned object, an electron-emitting device of the present invention includes:
first and second electrode layers;
a first insulating layer sandwiched between the first electrode layer and the second electrode layer;
an opening penetrating through the first electrode layer and the first insulating layer; and
an electron emitting material disposed in the opening and connected to the second electrode layer, and is characterized in that
a second insulating layer having an opening that is shaped in a taper such that an opening area on the first electrode layer side is larger than an opening area on the second electrode layer side is provided in the opening, and that
a third electrode layer having an opening disposed between the second insulating layer and the second electrode layer, and the electron-emitting material is formed inside the opening of the third electrode layer.
In addition, the electron-emitting device of the present invention includes:
first and second electrode layers;
a insulating layer formed between the first electrode layer and the second electrode layer;
a first opening disposed in the first electrode layer;
a second opening disposed in the insulating layer and communicating with the first opening; and
an electron-emitting film disposed in the second opening and connected to the second electrode layer, and is characterized in that
the second opening is shaped in a taper such that an opening area on the first electrode layer side is larger than an opening area on the second electrode layer side, and
an outer circumference of the electron-emitting film is sandwiched between the insulating layer and the second electrode layer.
In addition, the electron-emitting device of the present invention is characterized in that the electron-emitting material is a conductor.
In addition, the electron-emitting device of the present invention is characterized in that the second electrode layer, the third electrode layer and the electron-emitting material are at the same potential.
In addition, the electron-emitting device of the present invention is characterized in that the exposed surface of the electron-emitting material is positioned on a surface on a same level as the boundary of the second insulating layer and the third electrode layer or on the second electrode layer side.
In addition, the electron-emitting device of the present invention is characterized in that an opening shape of the opening is substantially circular.
In addition, the electron-emitting device of the present invention is characterized in that an opening shape of the opening is line-like.
In addition, the electron-emitting device of the present invention is characterized in that the first insulating layer and the second insulating layer are made of separate materials formed by different processes.
In addition, the electron-emitting device of the present invention is characterized in that a dielectric constant of the second insulating layer part is larger than a dielectric constant of the first insulating layer part.
In addition, the electron-emitting device of the present invention is characterized in that the third electrode layer and the electron-emitting material are formed of an identical material.
In addition, an electron source of the present invention is characterized in that a plurality of the above-mentioned electron-emitting devices are disposed.
In addition, the electron source of the present invention is characterized in that the plurality of electron-emitting devices are matrix-wired.
In addition, an image-forming apparatus of the present invention includes the electron source and an image-forming material for forming an image by electrons emitted from the electron source colliding with it.
In addition, the image-forming apparatus of the present invention is characterized in that the image-forming material is a luminous body for emitting light by the collision of electrons.